Unnecessary argument passed to method called by Thread.start

[drtwisted]

Godot version: 3.X

OS/device including version: Linux (Probably macOS & Windows as well)

Issue description:

When starting a thread using my_thread.start(self, "_run_thread") construction without userdata argument provided _run_thread is still called with argument which is equal to null, I assume it's because of the defualt value declared in Thread.start definition.

Steps to reproduce:
Run the following code:

func _run_thread():
    print("I am a thread")

func _ready():
   var thread = Thread.new()
   thread.start(self, "_run_thread")

Expected result:
_run_thread gets called and we see the output of "I am a thread"

Actual result:
Thread is not run because of an error:

ERROR: _start_func: Could not call function '_run_thread'' starting thread ID: 5 Reason: Too Many Arguments
   At: core/bind/core_bind.cpp:2286.

[vnen]

Duplicate of #9924